MadSci Network: Neuroscience

Re: Please explain the formation of the colors and the shapes in my spinning to

Date: Thu Jan 21 18:19:05 1999
Posted By: Tom Stickel, Grad student, Optometry, Indiana University School of Optometry
Area of science: Neuroscience
ID: 906003346.Ns

Hola Ms. Campos,
	You're question is a good one.  In fact, there are a few classical 
experiments on color that involve spinning tops. 
	There are two fundamental things you have to know to understand the 
answer to this question.  The first is the difference between "additive" 
color mixing and "subtractive" color mixing.  The second thing is a few 
basic facts about the visual system (good thing I'm in optometry school 
right now!).
	First off, when we think about mixing colors, what we normally think 
about is subtractive color mixing. OK, to start, we see a rose as red 
because it's absorbing almost all other colors but red, and reflecting 
mostly the red.  So if we mix red paint in with another color, the red paint 
is still absorbing everything but red.  If you mix in yellow paint (which 
reflects mostly yellow), the red and yellow are each still absorbing the 
same colors they were respectively absorbing before they were mixed.  
The red paint absorbs yellow, and the yellow absorbs the red. Between them, 
they "subtract" most other colors out.  But they both let a little orange 
reflect, so we perceive the paint as orange.
	Additive color mixing is a different kettle o' fish.  If you shine a 
red light on the same spot as a green light, you don't get gray (which is 
what you'd get by mixing them as paints).  You get something close to 
orange.  Why this happens has to do with the wave nature of light and other 
complicated stuff we don't need to get into.  Suffice to say, additive color 
mixing isn't something we usually think about.  A TV screen works by 
additive color mixing, for instance, but we usually don't wonder how it 
works.  There are many good web sites that let you do additive mixing to get 
a better idea how it works.  Here's one; for more, just type in "additive 
color mixing" in your favorite search engine:

Fun with 
additive colors

	Now, think about how many RPMs that top is doing.  It's a lot. Our eyes 
can't follow something that fast.  For instance, flourescent light bulbs are 
actually flickering, but it's occurring so fast that we can't see the 
individual flickers, but only a constant light. We know a movie is made up 
of individual  frames, but if you play it fast enough, it appears like 
smooth motion.  Our eyes and brains can't keep up with the separate images, 
so they fuse into a fluid image.  The fancy term for this is "temporal 
summation," that is, adding up over time. Same for your top. We can't see 
individual points on the top, because it's spinning too fast.  When it 
starts slowing down, you can start to see the individual circles again.
	Back to the top.  What you can do is take a compass and draw concentric 
circles  outward from the center of the top.  Now imagine you hold a needle 
over the top in a certain place and set the top spinning without moving the 
needle.  We also have to imagine that the top stays in the same place while 
spinning.  Now, if you look at the part of the top under the tip of the 
needle, you can see that every point on a given circle is spinning under the 
point of the needle many many times in the span of a second.  Your brain and 
eyes are too slow to see each point go by.  Just like the flickering light 
or the movie, your brain adds together all the information. Because the top 
is going so fast, every point on the circle gets added up and perceived at 
the same time.
	So hopefully now you can see that all you have to do is figure out what 
colors are on each circle and add them together with an additive color 
mixer!  You didn't describe the exact shape of the color wedges in the 
circles, but I imagine that on your inner circles you will find only red.  
The next circles out probably have some red points and some blue, which make 
violet.  The next circles are probably blue, then a mix of red and yellow 
(orange), then plain yellow, then yellow with maybe some blue.  The reason 
there is no green is that there is no single circle on the top that contains 
the right amounts of the three colors on your top to add together to make 
	But you're not just limited to tops!  Try the same thing with 
construction paper on anything that you can get to whirl rapidly (and 
safely), like a fan.  Try it with different colors and see what you can get.  
Try it with different colors taking up more or less of the fan.  You'll get 
some interesting results!
	If you have any more questions, email me at  Oh 
yeah, I forgot to thank Dr. Arthur Bradley, a very smart scientist here at 
the IU School of Optometry who explained most of this to me.

Good luck,
Tom Stickel

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